1. Field of the Invention
This invention relates to apparatus having demountable magnet systems and more particularly to experimental reactors for the investigation of the generation of power by fusion. The invention furthermore relates to coil structures and coil supports for fusion reactors and to methods pertaining to the same.
2. Prior Art
Toroidal field coils in a known nuclear reactor such as a Tokamak fusion reactor are interlaced with the blanket, OH and poloidal field coils thereof, as well as with neutral beam, coolant and vacuum lines, in a very complex arrangement. Not only does this make construction of the reactor cumbersome, but replacement of a failed toroidal field coil after construction would be extremely difficult, if not impossible. One illustration of this complexity of construction is shown by the ORNL EPR design ("Oak Ridge Tokamak Experimental Power Reactor Study Reference Design", ORNL, W-7405-Eng-26, 1975). Replacing a failed toroidal field coil in this reactor would involve removing and replacing a large number of radioactive massive components, with many cuts and seals to be made.
Of the many failure possibilities, three specific types of failure may be the following: in the first, failure of a toroidal field coil results in a non-operable reactor because it is impossible to replace the coil; in the second, the toroidal field coil can be replaced, but the reactor is down for a long interval (e.g., 3 years) while massive replacement operations, involving movement of a large portion of the blanket and shield structure, vertical field coils, etc., are carried out; in the third, the toroidal field coil is segmented (according to the invention as discussed hereunder) so that a failed segment can be replaced in a month without requiring operation on the rest of the reactor structure.
For purposes of analysis, each toroidal field coil is assumed to have an independent probability P.sub.1 of having a nonrepairable failure per year of operation. For example, a nonrepairable failure could result from arcing, breakage, etc., inside the superconducting winding. A vacuum leak in the toroidal field coil dewar might or might not be repairable, depending on the nature of the leak and the toroidal field coil design. For an operating reactor the number of failures of the toroidal field coil system, per operating year, assuming that the failure of one coil will shut down the reactor, is EQU P.sub.F =NP.sub.1 failures/operating reactor year (2.1)
where N is the number of toroidal field coils in the ractor. The reactor availability (percentage of time that the reactor operates) is then ##EQU1## where t.sub.R is the toroidal field coil replacement time in years. If the toroidal field coil cannot be replaced and the reactor must be permanently scrapped, t.sub.R is taken as 30 years, the normal expected life of the plant. In practice, a replacement reactor could be built in 10 years, but this would reflect an extra capital cost. In order to compare the costs of plant non-availability on a common basis, the plants are assumed to be built at a normal interval of 30 years.
A plant availability of 80% is considered good for present power plants, both fission and fossil. Not all the down-time can be allotted to toroidal field coil replacement in a fusion reactor, however, since other scheduled and non-scheduled replacement and maintenance operations will undoubtedly be necessary. Blankets will have to be replaced, beam and vacuum lines maintained, etc., and failure of non-fusion reactor components will also contribute to reactor down-time. As a minimum, a plant availability of 0.95 is required considering only toroidal field coil replacement. For an overall plant availability of 0.8, this would correspond to 25% of the shut down time being a result of replacement operations on toroidal field coils.
Toroidal field coil systems can be designed to permit replacement of entire coils. For example, each of the UWMAK-I toroidal coils (B. Badger, et al., "A Wisconsin Toroidal Fusion Reactor Design", UWFDM-68, University of Wisconsin, November, 1973) were designed to be removable along with their corresponding blanket and shield assembly and there were 12 such modules in the reactor. However, such design approaches tend to result in very massive modular sections for the reactor and require poloidal field windings outside the toroidal field coils of very large current capacity. In addition, massive amounts of inter-coil support structure would have to be moved, and joints between modules would have to be cut and remade. The time required for these operations could be quite long.
With respect to the prior art, no prior patents have been located which anticipate the proposals of the invention. Some of the prior art patents which have been located are U.S. Pat. Nos. 3,416,110; 3,742,408; and 3,978,442.
Robert Morris in U.S. Pat. No. 3,416,110 discloses a structure having to do with a cooling feature of the present invention. More particularly, there is disclosed in this patent an electrical transformer having a form-fit casing and a magnetic core-winding assembly disposed therein. The sides of the casing support and maintain the magnetic core. The top and bottom portions of the casing support the electrical winding assembly and restrain it against movement during short circuit stresses. The casing is completely filled with a fluid dielectric, which is force circulated through the winding assembly. External heat exchanger means are furthermore provided. As will be seen, this structure relates generally to certain features of the present invention but is not remotely concerned with the overall combination.
As will also be discussed hereinafter, the invention proposes, among other features, the construction of coils in the form of connected segments. J. Jaeger in U.S. Pat. No. 3,742,408 discloses with respect to an underwater connector an inductive coupling technique for a make and break connector. The connector is designed to operate on AC signals and uses potted toroids which are coupled respectively to a source of signals and to a load. The toroids are interconnected using single-turn loops. As is disclosed in this patent, the loops may be formed of segments which may be clamped together using a quick-disconnected means such as a wing nut or such as a top plate cooperating with a pair of wing nuts. As will become apparent hereinafter, this has no anticipatory effect with respect to the invention disclosed herein.
E. Spicar in U.S. Pat. No. 3,978,442 discloses how to avoid short circuiting in transformer windings. He states that it is a requirement that windings should at all times and independently of the temperature of the associated transformer by prestressed with certain compressive stress. He observes, moreover, that the plastic settling of the insulating material leads to a gradual decrease of the clamping force. According to the invention disclosed in this patent, a plate is provided on which the yoke of the transformer rests and between this plate and the coils there are arranged clamping boxes, the pressure in which is transmitted to the ends of the coils. Between the plate and the bottom wall of the transformer are furthermore provided generating boxes. The clamping and pressure generating boxes are connected for transmitting hydraulic pressure between the boxes. By arranging the pressure generating boxes below the active part of the transformer, a hydraulic pressure is obtained in such boxes which corresponds to the weight of the active part. This pressure is utilized in order to achieve a substantially constant clamping pressure on the windings independent of the settling occurring in the insulating material within the winding during the drying procedure or during field operation of the transformer. Thus, there is shown a method of supporting windings or the like with variable forces. This, however, is not anticipatory of the invention disclosed in the present application, as will become apparent hereinafter.
C. W. Bushnell, Plasma Physics Laboratory, Princeton University, Princeton, N.J. has disclosed in an article, The Toroidal Field Coils for the PDX Machine, the formation of a split coil design in which joints between conductor segments are formed with the use of copper or steel pins. Such joints, it is believed, will not be able to withstand the stresses induced by magnetic fields and thermal contraction in a superconductivity environment and may not afford the conductivity required for coils in fusion reactors.
F. A. Puhn et al, General Atomic Company, San Diego, Calif. discloses, in Design of Demountable Joint for Doublet III Toroidal Field Coil, a demountable finger joint at the top of a toroidal field coil. The coil, as in the previous article by Bushnell, consists of non-rectilinear segments and similarly employs connecting pins. These pins, which are of various types, would not, it is believed, be suitable for withstanding the stresses generated in a superconductor environment.
R. O. Hussung et al, Union Carbide Corp., Nuclear Division, Oak Ridge, Tenn., in ISX Toroidal Field Coil Design and Analysis, discloses an arrangement permitting removal of an upper segment of a toroidal field coil. The arrangement fails to take into account displacements due to low temperatures and high magnetic forces in the manner of the present application.